An installation of this type is disclosed in U.S. Pat. No. 5,244,489 to GRENIER, to which reference will be had for more details and the disclosure of which is incorporated herein by reference. In this document, the compressor for air supplied to the mixing column is driven by a low temperature turbine expanding the part of the air flow directed to the medium pressure column, into an arrangement requiring, to carry out the necessary compression, expanding with work a large portion of the air supplied to the medium pressure column, giving rise to losses of output and energy as well as overdimensioning of the means for refrigeration and purification of the air supplied to the distillation apparatus.